Materials having a low dielectric constant are sought after in various technical fields such as microelectronics or optics. In various applications, dielectric materials are sought which have, in addition, a low refractive index. Among the materials currently used, porous silicas have the lowest refractive index (1.2), bearing in mind that that of air is 1.002 and that of dense silica is 1.47.
Magnesium fluoride has a relatively low refractive index and various methods for preparing it are known.
A method is known from JP-05-105424 for preparing an antireflective film of MgF2, that consists in applying, to a substrate, a solution containing either an Mg precursor and an F precursor, or fine particles of MgF2. The Mg precursor may be a sulfate, a nitrate or a phosphate that may or not be hydrated, or an alkoxide. The F precursor is an alkali metal or quaternary ammonium fluoride.
Known from Fujihara, et al. [Scripta Mater. 2001, 44, 2031-2034] is a method for preparing MgF2 films that contain Ag nanoparticles via a sol-gel route. The method consists in preparing a solution of magnesium acetate in isopropanol and in adding thereto CF3COO2H and H2O, then silver acetate, in stirring the solution for 2 hours, then in applying it to a silica glass substrate by spin coating. Next, the coated substrate is subjected to a heat treatment at 300° C.-500° C. for 10 minutes in air, followed by quenching. The refractive index of the product obtained depends on the temperature of the heat treatment; it is 1.31 for 300° C., 1.29 for 400° C. and 1.25 for 500° C.
Known from Fujihara, et al. [Thin Solid Films, 2001, 389, 227-232] is a method for preparing MgF2 films containing ZnO nanoparticles via a sol-gel route. The method comprises a step of preparing a solution of magnesium acetate in isopropanol IPA with addition of CF3COO2H and H2O, a step of stirring the solution for 1 h, then keeping at 90° C. for one week to obtain an acetate precursor gel, and a step of dissolving the gel in IPA, then addition of Zn acetate and methanolamine and stirring of the solution, and a step of applying to a silica glass substrate by spin coating, then immediate heating at various temperatures between 300 and 500° C. for durations of 5 to 14 min depending on the samples. The refractive indices are similar to those mentioned above.